1. Field of the Invention
The present invention relates to a driving device which is applied to a stepping motor.
2. Description of the Related Art
There have conventionally been proposed various types of stepping motors for use as drive sources for various kinds of devices. A stepping motor according to a first prior art is reduced in the diameter from the center of the rotational axis thereof and is at the same time enhanced in output power (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. H09-331666).
FIG. 10 is an exploded perspective view of the stepping motor according to the first prior art, and FIG. 11 is a longitudinal cross-sectional view showing the internal construction of the stepping motor shown in FIG. 10, in an assembled state.
As shown in FIGS. 10 and 11, the stepping motor according to the first prior art is comprised of a rotor 201, a first coil 202, a second coil 203, a first stator 204, a second stator 205, an output shaft 206, and a connection ring 207.
The first stator 204 and the second stator 205 are formed of a soft magnetic material. The two rotors 204 and 205 are disposed in a manner opposed to each other in the axial direction of the stepping motor with a predetermined gap therebetween. The connection ring 207 is formed of a non-magnetic material, and holds the first stator 204 and the second stator 205 with the predetermined gap between the two stators 204 and 205. The output shaft 206 is rotatably held by a bearing 204E of the first stator 204 and a bearing 205E of the second stator 205. The rotor 201 is rigidly secured to the output shaft 206, and is formed by a magnet (permanent magnet) which is circumferentially divided into four sections which are magnetized such that they have alternately different poles.
As shown in FIG. 11, the first stator 204 has a foremost end which is comb-teeth-shaped, and includes first outer magnetic pole parts 204A and 204B which are opposed to an outer peripheral surface of the rotor 201 with a gap between the first outer magnetic pole parts 204A and 204B and the outer peripheral surface of the rotor 201, and first inner magnetic pole parts 204C and 204D which are opposed to an inner peripheral surface of the rotor 201 with a gap between the first inner magnetic pole parts 204C and 204D and the inner peripheral surface of the rotor 201. The second stator 205 includes second outer magnetic pole parts 205A and 205B which are opposed to the outer peripheral surface of the rotor 201 with a gap between the second outer magnetic pole parts 205A and 205B and the outer peripheral surface of the rotor 201, and second inner magnetic pole parts 205C and 205D which are opposed to the inner peripheral surface of the rotor 201 with a gap between the second inner magnetic pole parts 205C and 205D and the inner peripheral surface of the rotor 201.
The first coil 202 for magnetizing the first stator 204 is wound around the first inner magnetic pole parts 204C and 204D in a manner adjacent to the rotor 201 in the axial direction of the motor. The second coil 203 for magnetizing the second stator 205 is wound around the second inner magnetic pole parts 205C and 205D in a manner adjacent to the rotor 201 in the axial direction of the motor.
Rotation of the rotor 201 of the stepping motor is caused as follows: The energizing direction of the first coil 202 and that of the second coil 203 are switched to thereby switch the polarities of the first outer magnetic pole parts 204A and 204B, the first inner magnetic pole parts 204C and 204D, the second outer magnetic pole parts 205A and 205B, and the second inner magnetic pole parts 205C and 205D. This causes the rotor 201 to keep rotating.
In the stepping motor constructed as above, magnetic fluxes generated by energization of the first coil 202 and the second coil 203 flow from the outer magnetic pole parts to the inner magnetic pole parts radially opposed thereto, or alternatively from the inner magnetic pole parts to the outer magnetic pole parts radially opposed thereto, whereby the magnetic fluxes efficiently act on the rotor 201 (magnet) located between the outer magnetic pole parts and the respective associated inner magnetic pole parts. Further, the distance between each outer magnetic pole part and the associated inner magnetic pole part can be set to a value almost equal to the thickness of the hollow cylindrical rotor 201, and hence it is possible to reduce the resistance of a magnetic circuit formed by the outer magnetic pole parts and the inner magnetic pole parts. This makes it possible to generate a larger amount of magnetic flux with a smaller amount of electric current, which leads to the enhancement of output power of the stepping motor.
Another stepping motor according to a second prior art is a PM stepping motor (see e.g. Japanese Patent No. 3327406 and Japanese Laid-Open Utility Model Publication (Kokai) No. H05-091191). In the stepping motor, a stator part, a bobbin part, and a cover part having a hole for holding one bearing at a location corresponding to one end surface of the stator part are integrally formed of a synthetic resin.
The stator part includes two pairs of stator yokes coaxially arranged such that the inner peripheral surface of each pair of stator yoke form a cylindrical surface. The bobbin part defines a space between the collars of each of the two pairs of stator yokes, for holding a stator coil therein. The cover part is formed therethrough with the bearing fitting hole for holding a rotary shaft and the one bearing supporting the rotary shaft at the location corresponding to the one end surface of the stator part. A stator unit has a plurality of terminal pins protruded from the cover part in the same direction as the rotary shaft extends. A rotor is disposed within a hollow cylindrical part of the stator unit, and is formed by the rotary shaft and a magnet. An outer yoke is comprised of a flange having the other bearing rigidly secured thereto, and a casing fitted on the outer periphery of the stator unit. According to this stepping motor, it is possible to minimize coaxial deviation between the inner peripheries of the stator yokes and those of the bearings.
However, the stepping motor according to the first prior art necessitates provision of predetermined gaps between the inner peripheral surface of the rotor (magnet) and the outer peripheral surfaces of the inner magnetic pole parts opposed thereto. To control the manufacturing of the stepping motors such that the predetermined gaps are formed brings about an increase in manufacturing costs thereof. Further, although the stators are required to have a shape integrating hollow cylindrical inner magnetic pole parts and outer magnetic pole parts, it is difficult to integrally form the inner magnetic pole parts and the outer magnetic pole parts.
Further, in the case where the inner magnetic pole parts and the outer magnetic pole parts are separately formed, and then assembled together into one piece, the number of component parts increases, which results in an increase in the manufacturing costs. Moreover, since the axial dimension of the stepping motor is determined by the axial length of the two coils, that of the magnet, and the thickness of the stators, the stepping motor necessitates a large axial dimension so as to maintain predetermined output power thereof. On the other hand, if the axial length of the coils or the magnet is reduced, the output power of the stepping motor is considerably lowered.
Manufacturing of the stepping motor according to the second prior art requires the use of an insert molding machine as an apparatus for integrally forming the stator part and the bobbin part. This involves a significant increase investment in equipment, which affects manufacturing costs. In other words, manufacturing of the stepping motor having the stator part and the bobbin part integrally formed as a one-piece member necessitates large investment in equipment, as mentioned above, and hence it can be practical only for mass production.
Further, since the magnet is disposed inside the stator unit including the coils, the outer diameter of the magnet has to be smaller than that of the stepping motor. If the outer diameter of the stepping motor is reduced, that of the magnet is inevitably reduced, which causes significant reduction of the output power of the stepping motor.